Adam Hooper's Engineering Tips

Working With and Around FormBuilder

My current work is entirely focused on Ruby on Rails; and once you put Rails into focus, a lot of well-intended, badly-implemented designs peer back at you. Today’s topic: FormBuilder.

FormBuilder is a class most Rails users use transparently and most Rails users do not realize exists. It is the class of object returned by the form_for() and fields_for() helper methods.

Rails’s FormBuilder code looks like it was abandoned before being completed, and it suffers from abysmal documentation. Best practices, aside from being undocumented, do not cover all use cases—or even common use cases. In fact, best practices are downright nonexistent. Fortunately, all that can change with about 20 lines of well-placed code.

What FormBuilder does well

In your templates (depicted here in Haml instead of Rails’s default, ERb, as Haml is less sucky), FormBuilder shines with simple fields:

- form_for(@thing) do |f|
    = f.label(:name, 'Name:')
    = f.text_field(:name)
    = f.label(:description, 'Description:')
    = f.text_area(:description)

Here, f is the FormBuilder instance. It provides the label(), text_field() and text_area() methods.

What FormBuilder does badly

Inevitably, in the course of a project, you will want to use more than text fields and date fields. And here, FormBuilder falls flat. For instance, a select field might look like this:

    = f.label(:thing_type_id, 'Type:')
    =, ThingType.all(:order => :name).collect{ |t| [, ] }, :include_blank => 'None')

This verbose amalgamation of complicated code and simple markup is even, with no apparent embarassment, suggested in the Rails API documentation.

But even select() is elegant compared to other common solutions FormBuilder supposedly provides. Writing a functional date_field(), needed for virtually any serious Rails project, is a nightmare; compound fields are nonexistent; and, most common and worst of all, there is no way to specify, for instance, “title fields should look like this”: the very Don’t Repeat Yourself principle Rails fans so avidly embrace.

Mediocre Solutions

Rails plugins may ease the pain, but they almost never do exactly what you want. Even if you find a perfect date_field(), you are only postponing an inevitable need for a more general solution.

Helper methods can work around some of FormBuilder, but if you resort to your own system of helper methods, you end up essentially engineering your own mini-framework of form helpers. The problem here is the unintuitive divide (what is FormBuilder and what is custom?) and a very inconsistent method signature (that is, the types and orders of parameters). And if you do not plan things out ahead of time, your helper code which began as a helper or two will end as a confusing mess.

Developers also get tempted to use Rails’s alternate tagging library methods, such as text_field_tag(), select_tag(), and other non-incumbered helpers. The problem with these is that they do not do the wonderful things FormBuilder does: automatically figure out element names and IDs, for instance, magically calculate fields’ initial values, or (prior to Rails 2.2) output valid HTML. These methods should be avoided when building a form for a model.

The funny thing is, FormBuilder actually does solve all these problems, almost. It really does get 95% of the way there. This article presents the last 5%.

How FormBuilder works

FormBuilder, implemented mostly in action-pack/lib/action_view/helpers/form_helper.rb (but spread into date_helper.rb and form_options_helper.rb in a textbook example of “when not to use Ruby’s class-reopening feature”), is implemented with three main pieces:

First, the helper methods: ActionView::Helpers::FormHelper defines several methods which are available to every view: text_field(), text_area(), select(), and so on. Most of them follow a simple calling convention:

  def something_field(object_name, method, options = {})

And here’s the magical part: if options[:object] is set, the helper will call options[:object].send(method): it will use the actual object to determine the initial value of the field.

Second, ActionView::Helpers::FormBuilder (which is not a helper at all): essentially, FormBuilder holds the object you are building a form for and serves as a proxy to the helper methods. It redefines every helper method with its own version:

  def something_field(method, options = {})

FormBuilder uses its own object_name and object methods, so that when your template calls:

= f.text_field(:name, :size => 15)

FormBuilder (in the above example, f) passes the call through as:

= text_field(f.object_name, :name, :size => 15, :object => f.object)

Lastly, InstanceTag: an implementation detail which may strike you at first as over-engineered, but upon further inspection rubs off as sideways-engineered at best and upside-down-engineered the rest of the time. Try to avoid calling its methods or modifying it whenever possible.

What We Want

We want a simple way to add new types of fields to FormBuilder: perhaps, extending the above examples, a thing_type_field() or a date_field(). I will showcase both below.


I shy away from plugins and I encourage others to act the same way. If you do not understand this code, you should not use it: that would be applying my solutions to your problems, which only works if you have the exact same problems as I do, which is unlikely. Continue on your merry way until you reach the aforementioned impasses, and use this guide as an aid in solving your problems.

That said, here is my solution:

Create the directory lib/forms/ and add a file, lib/forms/application_helper.rb, and leave it nearly empty for now:

module Forms::ApplicationHelper
  # Helper methods go here

At the top of app/helpers/application_helper.rb, include it:

module ApplicationHelper
  include Forms::ApplicationHelper
  #... leave whatever was here before ...

Create a custom FormBuilder, in lib/forms, in a file called lib/forms/application_form_builder.rb:

class Forms::ApplicationFormBuilder < ActionView::Helpers::FormBuilder
  # Copied from FormBuilder. FormBuilder looks like it has some bright
  # engineering ideas but never finished implementing them. This *should*
  # be automated by defining "self.field_helpers", but it's used before
  # this class is loaded.
  Forms::ApplicationHelper.instance_methods.each do |selector|
    src = <<-end_src
      def #{selector}(method, options = {})
        @template.send(#{selector.inspect}, @object_name, method, objectify_options(options))
    class_eval src, __FILE__, __LINE__
  def objectify_options(options)
    @default_options.merge(options.merge(:object => @object))

The above code is copied, almost word-for-word, out of Rails’s original FormBuilder.

Finally, we want to use our own ApplicationFormBuilder instead of Rails’s default, so add the file config/initializers/application_form_builder.rb with the following contents:

ActionView::Base.class_eval do
  def self.default_form_builder

Restart your Rails application (this is the last time—a subtle bonus) and we are now using our new-and-improved ApplicationFormBuilder. At the moment, it behaves exactly like Rails’s regular FormBuilder: it maintains all the great features, adding no overhead. But now we can extend it!

A better date_field

When engineering, always determine what the customer wants (which should be easy here, because you are the customer). In my case, I want to write this:

  = f.label(:start_date, 'Start Date:')
  = f.date_field(:start_date)

Now, let’s make this work!

I use jQuery for all my JavaScript needs, which is fantastic because it lets me work without HTML IDs and it lets me remove 100% of my JavaScript code from my templates, which is good because code does not belong in templates. There are a couple of jQuery date-select plugins which border on acceptable; let us use the one which comes as part of the jQuery-UI download, ui.datepicker.js.

Our first hurdle: all these JavaScript date-picker fields submit text (for example, "Nov 10, 2008") which the model does not expect. We will need to edit our model to allow reading and writing a text representation. So let’s take our model, for instance, app/models/thing.rb

class Thing
  # ... typical model stuff ...

  def start_date_string
    d = self.start_date
    d.blank? ? nil : d.strftime('%b %d %Y')

  def start_date_string=(value)
    d = nil
      d = Date.parse(value)
    rescue ArgumentError
      # d = nil

    self.start_date = d

(In a real project, this behaviour should be extracted into a mix-in.)

Now we have Thing::start_date_string() and Thing::start_date_string=(), which behave like Thing::start_date() and Thing::start_date=() except they allow human-readable strings that can be submitted via HTML forms.

We should add the necessary JavaScript includes, as well, in app/views/layouts/application.html.haml (or, as the case may be, app/views/layouts/application.html.erb—or whatever layout you happen to be using), in the header:

= javascript_include_tag(['vendor/jquery.js', 'vendor/jquery.ui.all.js', 'date_field.js'])

To tie our HTML into jQuery (since, as mentioned above, we write no JavaScript in our HTML), our templates will need to output text fields with a special class attached. Let us arbitrarily choose the HTML class, date_field.

Create public/javascripts/date_field.js with the following contents:

$(document).ready(function() {
    // Add options here...
    dateFormat: 'M d yy'

This JavaScript, when loaded, will turn any HTML input with class date_field into an interactive date selector.

Now, let us use our magical ApplicationFormBuilder to create the missing piece of the puzzle, by adding the following to lib/forms/application_helper.rb:

  def date_field(object_name, method, options = {})
    options = options.reverse_merge(:size => 12)
    if options[:class].blank?
      options[:class] = 'date_field'
      options[:class] += ' date_field'
    method = "#{method}_string".to_sym
    text_field(object_name, method, options)

Presto! Your template should work now. If you have forgotten, your template looks like this:

- form_for(@thing) do |f|
    = f.label(:start_date, 'Start Date:')
    = f.date_field(:start_date)

And internally, that behaves as follows:

  1. The template calls form_for(@thing) to create a new ApplicationFormBuilder, f. (The initializer we wrote specifies ApplicationFormBuilder instead of the regular FormBuilder.)
  2. The template calls f.date_field(:start_date) (and maybe passes optional parameters, such as an HTML class or style). ApplicationFormBuilder::date_field() is not the helper you wrote: it is a new method, generated automatically because you wrote the helper method.
  3. ApplicationFormBuilder::date_field() calls the helper you wrote, date_field(:thing, :start_date).
  4. Forms::ApplicationHelper::date_field() calls text_field(:thing, :start_date_string, :object => thing, :size => 12, :class => 'date_field')
  5. FormHelper (the Rails helper) creates the text field, with the date_field HTML class.
  6. When the page loads, jQuery selects all text fields with the date_field HTML class and calls datepicker() on them, turning them into date pickers.

Whew, a lot of work, right? yes. But here are the advantages:

  • We can pick another jQuery-style date-picker implementation, and all we need to change is public/javascripts/date_field.js.
  • We can change the HTML completely, for instance reverting to Rails’s three-dropdown-box behaviour, by simply changing lib/forms/application_helper.rb.
  • It is easy to unit-test the helper, the template, and the JavaScript, since each has an isolated and distinct role.
  • Views receive the real benefits:
    • There is no bloat in a simple call to f.date_field(:method): we remain readable and concise.
    • Site-wide changes to date field behaviour can transpire without a need to edit templates.

As for drawbacks? This procedure is certainly complicated, so some people may prefer to just install a Rails plugin and be done with it. I have tried a few date selector plugins and found them all lacking in one way or another compared with this implementation, but your mileage may vary.

A better select()

While ranting above about FormBuilder shortcomings, I produced this piece of code, which the Rails API documentation shockingly presents as exemplary.

    = f.label(:thing_type_id, 'Type:')
    =, ThingType.all(:order => :name).collect{ |t| [, ] }, :include_blank => 'None')

What do we, the customer (template writer), want to type? Certainly not all that, since we may have ThingType selectors elsewhere in our application and we are almost guaranteed to need to change the code within as our application evolves. Helper methods are a common but unsatisfactory compromise. What we really want is this:

    = f.label(:thing_type_id, 'Type:')
    = f.thing_type_field(:thing_type_id)

And the implementation? Easy: Just add this to lib/forms/application_helper.rb:

  def thing_type_field(object_name, method, options = {})
    select(object_name, method, ThingType.all(:order => :name).collect{ |t| [, ] }, options.reverse_merge({:include_blank => 'None'}))


  • Our template is simple.
  • We reuse code and ensure consistency across the site by only writing our model query once.
  • Because the model query code is now in a helper (where it belongs) and not in the template (where it does not), it can (and should) be easily split into multiple lines and have more logic added.


  • We lose the nice html_options parameter in the select() method’s signature, which is inconsistent across the Rails API but useful nonetheless. It is straightforward to pass a :html_options item within our options hash to work around this problem.

A Nested Object Field

Rails is unhelpful when you want to include a nested object in your form. The best you can do is render a partial, but I find that annoying because it melds interface with implementation. How about this:

  def foo_field(object_name, method, options = {})
    foo = options[:object] && options[:object].send(method) ||
    fields_for("#{object_name}[#{method}]", foo) do |f|
      render(:partial => '/foo/form', :object => f)

And because at this point all your form partials use FormBuilder exclusively and never rely upon an @object or @foo or anything like that, this works without a hitch.

Parting Thoughts

This guide is the result of many of hours of hard work with FormBuilder, and I hope it helps save others that time. If you have any questions, comments, or even completely different and possibly better solutions, I would love to hear them.